Reimplement accessor documentation merging.

This time, merging handles both writers and setf expanders.

2013-06-19  Didier Verna  <didier@didierverna.net>

	* src/util/texi.lisp (@defunx):
	(@defun): Reimplement in terms of @deffn and @deffnx.
	* src/doc/symbol.lisp (document (accessor-definition)):
	Reimplement merging.
	(document (defsetf-definition)): Use "reader" and "writer" instead
	of "access" and "update".